Project Summary Tonic inhibition is a major form of inhibition in the central nervous system (CNS) and could play a key role in epileptogenesis and seizure generation. Nonsynaptic GABAA receptor (GABAAR) subunits that mediate tonic inhibition, including the ? subunit, are frequently altered in animal models of lesion-induced epilepsy. However, it remains unclear whether several of these changes contribute to or limit seizure activity. It becomes particularly important to understand the functional effects of these changes as new pharmacological agents that can modify the function of ? subunit-containing receptors are developed. A set of new technologies, reagents and mice will be used to study the functional effects of altering GABAAR subunit expression in granule cells of the dentate gyrus which are considered to be major players in seizure generation or propagation within the hippocampal network. The proposed studies will use Cre recombinase-dependent viral vectors to express GABAAR ? subunit selectively in granule cells and determine the effects of this alteration on epileptiform activity within the network in a mouse model of epilepsy. A combination of light and electron microscopic immunohistochemical methods will be used to evaluate changes in GABAAR subunit expression, and the functional correlates will be studied with electrophysiological methods, behavioral memory tasks, and Fos expression as a marker of neuronal activity in vivo. The broad goal of the project is to determine if increasing nonsynaptic GABAAR subunits in a cell-type selective manner will influence tonic inhibition and epileptiform activity in a mouse model of epilepsy. Specific Aim 1 will test the hypothesis that transfection of the ? subunit in dentate granule cells will rescue the decreased expression of this subunit that frequently occurs in mouse models of epilepsy and, in the process, normalize expression of two related subunits, ?4 and ?2, that are also altered in mouse models of epilepsy. Specific Aim 2 will test the hypothesis that an increase in ? subunit expression will enhance GABAAR-mediated tonic inhibition in dentate granule cells and reduce heightened responses of these neurons to perforant path stimulation in seizure-prone mice. Specific Aim 3 will test the hypothesis that increasing ? subunit expression in dentate granule cells will reduce network hyperexcitability and propagation of epileptiform activity in an animal model of epilepsy, using a novel hippocampal-entorhinal cortex slice preparation. Specific Aim 4 will test the hypothesis that enhancing ? subunit expression in dentate granule cells will lead to improved performance on a working memory task in which pilocarpine-treated mice are normally deficient. The effects of increased ? subunit expression on granule cell activity in vivo during an elicited seizure will also be determined. Together these studies will increase our understanding of the effects of altering ? subunit-mediated tonic inhibition selectively in dentate granule cells on hippocampal network function in an animal model of epilepsy. The results could suggest GABAAR subtype and neuron-specific targets for reducing seizure activity and associated cognitive deficits.